An audio generation device allows restoral of the analogue audio signal of a digital sound sequence. It uses a bank of digital samples of sounds, and an audio conversion chain, capable of delivering, to a loudspeaker, an analogue audio signal corresponding to a determined sequence of digital samples of the bank, which corresponds to a voice or sound message to be emitted.
Such a device is schematically represented in FIG. 1. It comprises a control circuit 1, which receives the requests to emit sound messages, a data bank 2 containing digital samples Dj and a digital analogue conversion audio chain which delivers an analogue audio signal Si to a loudspeaker 3.
On receipt of a control signal CG(MSGi) instructing audio generation of a determined message MSGi, the control circuit 1 establishes the series Sqi of digital samples Dj corresponding to this message MSGi, activates the audio chain 10, typically by means of an activation signal EN, and supplies this audio chain with the series of samples Sqi, at the working frequency of the converter, typically equal to the sampling frequency.
The audio chain 10 provides as output an analogue signal Si, to the loudspeaker 3 which delivers the requested message.
The bank of samples can comprise samples of sound sequences constructed digitally, corresponding for example to sound beeps (mono or multi-frequency digital generators), or obtained by digital sampling at an appropriate frequency, of a recorded audio analogue signal. Typically, a sampling at a frequency of the order of 16 kilohertz and an amplitude coding on 16 bits are suitable for the envisaged application.
The digital analogue conversion chain typically comprises a digital analogue converter 11 and an analogue adaptor 12, typically an audio amplifying filter. By nature, the sound messages restored have variable characteristics (frequencies, amplitudes, durations).
In practice the digital circuitry (circuits 1, 2, 11) operates at the same tempo as a clock signal CK. The frequency of this clock signal corresponds in practice to the sampling frequency of the samples Dj, i.e. typically 16 KHz, in the case of the above example for instance.
In an aircraft, the use of an audio generation device (or of several) by the alarm management system in order to make voice announcements to the pilot, for example a fault detection announcement, or aircraft altitude announcements in the descent phases, is advantageous in that it allows the pilot to concentrate on pure piloting, and that he is capable of reacting immediately and directly, to the announcement of such or such a message, by corresponding piloting actions.
Such a use relating to the management of alarms nevertheless assumes that the integrity of these audio generation devices can be guaranteed. It must be possible to guarantee this integrity at any moment. It must notably be possible to continuously verify proper operation of the function for applying a sequence of samples as input to the digital analogue converter, on activation of a corresponding control signal by the system for generating the alarms and the proper operation of the whole of the conversion chain, namely the digital analogue converter and the amplifier.
More particularly, the usual faults to be detected are an erroneous working frequency, for supplying the converter with samples, or the decoding operation in the converter, which would be manifested through an error in the spectrum of frequencies of the analogue signal restored at output, or errors of amplitude decoding in the converter or errors of analogue adaptation in the amplifier, which would be manifested through an error in the amplitude of the analogue signal restored at output.
Depending on the degree of reliability sought, other errors have to be detected, such as the unexpected emission of sound announcements, that is to say outside of any instruction from the system for generating alarms, corresponding for example to the noise generated by a defective amplifier, or the non-emission of a sound announcement, although it has been instructed.
However, sound messages being diverse, the characteristics of the analogue signal delivered at the output of the conversion chain are variable, unpredictable, both as regards amplitudes, and frequencies, or durations.
Hence, the design of an integrity monitoring system for such a device, which would be based on an analysis of the analogue signal at output, is not easy.